This invention relates to a method and a system for demodulating receive signal including pilot signal, and more particularly to, a method and a system for demodulating receive signal including pilot signal while suppressing BER (bit error rate) on the basis of unique word as pilot signal in mobile satellite communication incurring a fast fading on transmission path.
FIG. 1 shows a mobile terminal for mobile satellite communication. This mobile terminal, for example, a portable telephone, is composed of an antenna 81 to receive a transmit signal with a pilot signal in multiple blocks through a satellite, a signal converter 82 to convert the received signal from analogue to digital, a demodulation circuit 10 to demodulate the converted signal, a voice signal generator to convert the demodulated signal from digital to analogue, and a speaker 84 to output a voice signal.
In FIG.1, a receive signal including a pilot signal received by the antenna 81 is converted into a digital signal. The converted digital signal is demodulated by the demodulation circuit 10, and then converted into an analogue voice signal by the voice signal generator 83. The analogue voice signal converted by the voice signal generator 83 is output as voice sound from the speaker 84.
In the above satellite communication system, for the purpose of reducing required power or miniaturizing the antenna, further for the purpose of keeping BER low even under the condition with a low signal-to-noise power ratio, an error correction code with large encoding gain or a high-efficiency encoding technique is used.
FIG.2 shows a demodulation system for continuous wave, which is described in Meyr, xe2x80x9cDigital Communication Receiverxe2x80x9d, John Wiley andSons, pp.744-747, 1997, that can be used as the demodulation circuit 10 in FIG. 1. In this system, for a multipath fading including frequency-selective fading and flat fading with no frequency characteristic, receive signals are demodulated compensating a distortion in receive signal on flat fading (hereinafter simply referred to as xe2x80x98fading distortionxe2x80x99).
In FIG. 2, a continuous-wave receive signal with unique words as pilot signal inserted at given intervals is input to an input terminal IN, and then supplied to a delay circuit 91 and a unique word demodulator 93. A unique word in the receive signal is demodulated by the unique word demodulator 93, input to a data distortion estimator 94 composed of a Wiener filter. The data distortion estimator 94 estimates the fading distortion of data in the receive signal based on the fading distortion of unique word calculated from known values (e.g., 0 that +1 is phase-modulated, and xcfx80 that xe2x88x921 is phase-modulated) and the value of the demodulated unique word, then outputting it to a data demodulator 92. The data demodulator 92 demodulates data delayed a given time by the delay circuit 91 while compensating it with the estimation quantity of fading distortion, then outputting a demodulated signal from an output terminal OUT. Thus, data transmitted on continuous wave can be demodulated with the fading distortion compensated.
FIG. 3 shows another demodulation system, which is described in M. Sanpei, xe2x80x9c16QAM Fading Distortion Compensation Method for Land Mobile communicationxe2x80x9d, IEICE Paper Publication B-11, Vol. J72-B-11, No. 1, pp. 7-15, 1989, that can be used as the demodulation circuit 10 in FIG. 1. This system demodulates compensating the fading distortion of a continuous receive signal with one unique word symbol inserted for every (Nxe2x88x921) information symbols. In FIG.3, when a receive signal is input to an input terminal IN, fading distortion estimators 111, 112 and 113 calculate the fading distortion estimate values c(kxe2x88x921), c(k) and c(k+1) of (kxe2x88x921)th, k-th and (k+1)th unique words in the receive signal that are delayed in the sequence. The estimate values are multiplied with 0th-order or 1st-order interpolation coefficients by multipliers 114, 115 and 116, and added by an adder 117. The adder 117 outputs c{k+(m/N)} as the fading distortion of m-th information symbol in k-th information symbol sequence, as the addition result. The fading distortion of this information symbol is transformed into 1/c{k+(m/N)} by a reciprocal transformer 118, then output to a multiplier 120. The multiplier 120 multiplies information symbol in the receive signal delayed by a delay circuit 119 by 1/c{k+(m/N)} output from the reciprocal transformer 118 to demodulate its information, which is output from a output terminal OUT. Thus, the receive signal is demodulated compensating the envelope curve and phase of the receive signal distorted by fading distortion.
However, the demodulation system in FIG. 12 has a problem described below. It can demodulate the receive signal at a desired BER for continuous wave that unique words are inserted at given intervals to continuous data. But, for example, when receiving a burst signal with frame format that unique words are inserted to the several parts of a 100 to 150 symbol data sequence, since the fading distortion of data sequence at both ends is estimated from the fading distortion of unique word at one side, the estimation precision of fading distortion of data located at the opposite side of the unique word is reduced. Therefore, under the condition with fast fading or low C/N ratio, a desired BER, e.g., such a BER that corresponds to a deterioration 0.5 dB of from theoretical value when the Rician fading intensity is 0 dB and the Doppler frequency is 0.01%, cannot be obtained. If instead of the Wiener filter, a Kalman filter or MAP estimation with a higher estimation precision is used, since it is such a matrix-like technique that requires a large amount of operation, the amount of operation increases.
For the demodulation system in FIG. 3, when it uses a 0th-order interpolation coefficient, one unique word value is retained over the corresponding (Nxe2x88x921) information symbols. Also, when it uses a 1th-order interpolation coefficient, the fading distortion of information symbol is estimated by connecting between adjacent unique words with a straight line. Therefore, under the condition with fast fading or low C/N ratio, a desired BER cannot be obtained.
Accordingly, it is an object of the invention to provide a method and a system for demodulating receive signal including pilot signal that even when fast fading occurs on the channel, the fading distortion can be estimated at a high precision and a low BER can be obtained, without increasing the amount of operation.
It is a further object of the invention to provide a method and a system for demodulating receive signal including pilot signal that in receiving a burst signal, a low BER can be obtained compensating the fast fading.
According to the invention, a method for demodulating receive signal including pilot signal that a receive signal is demodulated estimating the fading distortion of data by detecting the fading distortion of unique word as pilot signal inserted into multiple sections of the receive signal, comprises the steps of:
comparing a unique word in the multiple sections of receive signal and a known unique word and thereby detecting the fading distortion of unique word in the multiple sections;
calculating a spline interpolation curve based on the fading distortion of unique word in the multiple sections;
estimating the fading distortion of data in the receive signal from the spline interpolation curve; and
demodulating the data in the receive signal based on the fading distortion of the data.
According to another aspect of the invention, a method for demodulating receive signal including pilot signal that a receive signal is demodulated estimating the fading distortion of data by detecting the fading distortion of unique word as pilot signal inserted into multiple sections of the receive signal, comprises the steps of:
comparing a unique word in the multiple sections of receive signal and a known unique word and thereby detecting the fading distortion of unique word in the multiple sections;
calculating a spline interpolation curve based on the fading distortion of unique word in the multiple sections;
estimating the first fading distortion of data in the receive signal from the spline interpolation curve;
estimating the second fading distortion of data in the receive signal by inputting the fading distortion of unique word in the multiple sections to a Wiener filter; and
demodulating the data in the receive signal based on the first and second fading distortions of the data.
According to another aspect of the invention, a method for demodulating receive signal including pilot signal that a receive signal is demodulated estimating the fading distortion of data by detecting the fading distortion of unique word as pilot signal inserted into multiple sections of the receive signal, comprises the steps of:
comparing a unique word in the multiple sections of receive signal and a known unique word and thereby detecting the first-stage fading distortion of unique word in the multiple sections;
calculating a first-stage spline interpolation curve based on the first-stage fading distortion of unique word in the multiple sections;
estimating the-first-stage fading distortion of data in the receive signal from the first-stage spline interpolation curve;
demodulating the receive signal based on the first-stage fading distortion of data in the receive signal and then outputting a demodulated signal;
comparing the demodulated signal with a predetermined threshold value and thereby outputting a binary signal;
comparing a unique word in the multiple sections of receive signal and a known unique word and thereby detecting the second-stage fading distortion of unique word in the multiple sections;
comparing data adjacent to unique word in the multiple sections with the binary signal and thereby detecting the fading distortion of adjacent data;
calculating a second-stage spline interpolation curve based on the average of the second-stage fading distortion of unique word in the multiple sections and the fading distortion of adjacent data;
estimating the second-stage first fading distortion of data in the receive signal from the second-stage spline interpolation curve;
estimating the second-stage second fading distortion of data in the receive signal by inputting the average of the second-stage fading distortion of unique word in the multiple sections and the fading distortion of adjacent data to a Wiener filter; and
demodulating the data in the receive signal based on the second-stage first and second fading distortions of the data.
According to another aspect of the invention, a system for demodulating receive signal including pilot signal that a receive signal is demodulated estimating the fading distortion of data by detecting the fading distortion of unique word as pilot signal inserted into multiple sections of the receive signal, comprises:
detection means for comparing a unique word in the multiple sections of receive signal and a known unique word and thereby detecting the fading distortion of unique word in the multiple sections;
spline interpolation estimation means for calculating a spline interpolation curve based on the fading distortion of unique word in the multiple sections, and estimating the fading distortion of data in the receive signal from the spline interpolation curve; and
demodulation means for demodulating the data in the receive signal based on the fading distortion of the data.
According to another aspect of the invention, a system for demodulating receive signal including pilot signal that a receive signal is demodulated estimating the fading distortion of data by detecting the fading distortion of unique word as pilot signal inserted into multiple sections of the receive signal, comprises:
detection means for comparing a unique word in the multiple sections of receive signal and a known unique word and thereby detecting the fading distortion of unique word in the multiple sections;
spline interpolation estimation means for calculating a spline interpolation curve based on the fading distortion of unique word in the multiple sections, and estimating the first fading distortion of data in the receive signal from the spline interpolation curve;
Wiener filter means for estimating the second fading distortion of data in the receive signal from the fading distortion of unique word in the multiple sections; and
demodulation means for demodulating the data in the receive signal based on the first and second fading distortions of the data.
According to another aspect of the invention, a system for demodulating receive signal including pilot signal that a receive signal is demodulated estimating the fading distortion of data by detecting the fading distortion of unique word as pilot signal inserted into multiple sections of the receive signal, comprises:
first detection means for comparing a unique word in the multiple sections of receive signal and a known unique word and thereby detecting the first-stage fading distortion of unique word in the multiple sections;
spline interpolation estimation means for calculating a first-stage spline interpolation curve based on the first-stage fading distortion of unique word in the multiple sections, estimating the first-stage fading distortion of data in the receive signal from the first-stage spline interpolation curve;
first demodulation means for demodulating the receive signal based on the first-stage fading distortion of data in the receive signal and then outputting a demodulated signal;
decision means for comparing the demodulated signal with a predetermined threshold value and thereby outputting a binary signal;
second detection means for comparing a unique word in the multiple sections of receive signal and a known unique word and thereby detecting the second-stage fading distortion of unique word in the multiple sections, comparing data adjacent to unique word in the multiple sections with the binary signal and thereby detecting the fading distortion of adjacent data, outputting the average value of the second-stage fading distortion and the fading distortion of the adjacent data for each of the multiple sections;
second spline interpolation estimation means for calculating a second-stage spline interpolation curve based on the average values in the multiple sections, estimating the second-stage first fading distortion of data in the receive signal from the second-stage spline interpolation curve;
Wiener filter means for estimating the second-stage second fading distortion of data in the receive signal from the average values in the multiple sections; and
second demodulation means for demodulating the data in the receive signal based on the second-stage first and second fading distortions of the data.